Currently, commercial aircraft employ guidance systems that warn pilots when the aircraft is deviating from a flight path. Guidance systems must have certain levels of accuracy, integrity, continuity, and availability during ordinary flight. Guidance systems that are used for landing require additional levels of accuracy, integrity, continuity, and availability. Landing systems typically provide high precision data relating to position and deviation of an aircraft from a landing approach path. This high precision often requires special equipment, which can be beneficial in situations where fog, clouds, and/or other conditions reduce visibility.
Airport landing systems are categorized by the Federal Aviation Administration (FAA) or other certification authority into different categories (Category I, II, and III) depending upon levels of accuracy, integrity, continuity, and availability provided by the landing guidance system. Accuracy refers to a volume that a position fix is contained within ninety-five percent certainty. Integrity refers to the probability that the system will unintentionally provide hazardous misleading information, such as an undetected fault or lack of information. Integrity also refers to a time required for a detected fault to be flagged by the system. Continuity refers to the probability that the navigation accuracy and integrity requirements will remain supported during the approach.
Most airport landing systems fall in Category I (CAT I), which enables the aircraft to initiate approach procedures from a decision height (DH) of 200 feet. The decision height represents the lowest altitude, above the touchdown zone; the aircraft can descend to without the pilot making visual contact with the runway. In a CAT I landing, if the pilot has not made visual contact with the runway by the time the aircraft descends to 200 feet, then the pilot must abort the landing and try again. Also, for a CAT I landing, the plane has to be in a runway visual range (RVR) of at least 1800 feet, which means that the pilot must also make visual contact with the start of a center line of a runway with no less than 1800 feet to the runway. In other words, if the aircraft attempting a CAT I approach is located at least 200 feet above the runway (DH) and at least 1800 feet from the start of the runway (RVR) and the pilot is able to make visual contact with the runway by that point, then the aircraft can continue with the CAT I approach. Otherwise, the aircraft should abort the landing.
More restrictive than the CAT I landing is a Category II (CAT II) landing, where airport landing systems allow the aircraft to initiate approach procedures from a DH of at least 100 feet and a RVR of at least 1200 feet. An aircraft that is capable of a CAT II landing is able to descend below the CAT I landing requirements before making a decision whether to land or abort the landing. In a CAT II approach, the DH is located at least 100 feet above the runway and the RVR is at least 1200 feet from the start of the runway.
Airport landing systems categorized for CAT III, like the system currently found at John F. Kennedy International Airport, allow for landing procedures from a DH of at least 50 feet and a RVR of at least 650 feet. In an aircraft capable of a CAT III approach, the DH is located at least 50 feet above the runway and the RVR is at least 650 feet from the start of the runway.
Aircraft configured for CAT III landings require special automatic landing or guidance systems, such as a triple redundant autopilot system, and must meet stringent levels of integrity and reliability. Generally, only a few airports have the equipment necessary for CAT III landings because implementation of such equipment requires special surveying. In addition, limited aircraft crews have the requisite training to perform the CAT III landings, such as the requisite simulator training, for example. Due to these limitations, CAT I landing systems and approaches tend to be the predominant methods used in smaller or private airplanes.
One of the landing systems used throughout the world for high precision landing guidance and deviation data is an instrument landing system (ILS), which includes a transmitter located on the ground to project two sets of radio beams into space along the approach corridor. An aircraft equipped for an ILS landing includes specialized antennas and receivers that interpret the radio beams and provide the pilot with navigational guidance. One of the radio beams provides lateral guidance, which allows the pilot to align the aircraft with the runway. A subsystem associated with the lateral guidance is called the localizer. The other radio beam provides vertical guidance. The subsystem associated with the vertical guidance is called a glideslope and it provides guidance for a steady descent into the airfield. The combination of the localizer and the glideslope effectively defines an approach path for an aircraft to fly along during a landing. The approach path is often referred to as an ILS approach. Depending on the configuration and equipment used, ILS is capable of CAT I, II, and III landings.
Another guidance system used for landings is a Wide Area Augmentation System (WAAS). WAAS is a system using both a ground-based component and a GPS satellite component in order to determine both the lateral and vertical position of the aircraft during a landing approach. The ground-based component may comprise a number of dispersed ground monitoring stations, while the GPS satellite component may comprise a constellation of between twenty-four and thirty-two Medium Earth Orbit satellites. The satellites transmit precise microwave signals that are received by GPS receivers on an aircraft to determine current location, time, and velocity of the aircraft.
The navigational data provided by a WAAS is used with a Localizer Performance with Vertical Guidance (LPV). The LPV is a high precision GPS (WAAS enabled) aviation instrument approach procedures that assists in determining a lateral position and a vertical position of the aircraft. Similar to an ILS approach, the LPV defines the approach path (referred to as an LPV approach) for the aircraft to fly during an approach to a given airport. The LPV approaches (for airports that have defined LPV approaches) are contained in a database that is used by the aircraft and the WAAS to generate deviation and guidance data for an LPV approach. In most cases, the FAA has defined the LPV approaches such that they match existing ILS approaches. Currently, WAAS is only qualified for a CAT I landing and is currently unable to execute by a CAT II or III landing.